Can only add a single bone in object mode
This is expected behaviour. You can also just add one cube at a time. It is one object.

Depending of what you want to do, you could parent the first armature to the second armature.

There are several parenting methods available in the object menu. You could parent the whole armature as one object. Or to the bone of the second armature.

To be able to do parenting, first select the first armature (this will be the child), then hold down shift and select the second armature ( this will be the parent). That way both objects are selected. Then choose your parent method.